Fix namespace to exclude ApiPlatform test HttpClient from instrumentation (#277)

Author: ralfkuehnel

src/HttpClientInstrumentation.php

@@ -24,7 +24,7 @@ final class HttpClientInstrumentation
      */
     const SYNCHRONOUS_CLIENTS = [
         /** @psalm-suppress UndefinedClass */
-        '\ApiPlatform\Symfony\Bundle\Test\Client',
+        'ApiPlatform\Symfony\Bundle\Test\Client',
     ];
 
     public static function supportsProgress(string $class): bool